Buying solar panels can be a daunting process, even for those who have a good understanding. If you don’t know the first thing about sizing and selecting the right panel for your needs, then keep reading.
In this article I will outline the basics behind what to look for and how to make sure you have enough solar to cover all your 12v needs, without spending too much. First thing’s first. Let’s get some terminology clear for 12 volt newbies. DC power is what you get from your 12v battery in your vehicle/caravan. AC power is what a power point at home or an 12v/230v inverter gives you.
When calculating solar panel sizes, you have 3 things to be aware of. Watts, amps and volts. For the 12v newbies:
What are you running, and when?
Now, before you even start looking at solar panels, you not only need to determine what appliances you want to run, but also when you need to run them, i.e. day use vs night use. This is not only critical when sizing your solar, but also your batteries. Solar tops you up during the day and your battery keeps you going at night. There’s no point having a big battery, but not enough solar to charge it when it runs down, or worse, having massive solar panels, but nowhere to store it for your night time use.
Here’s a few things to think about. Think about anything that runs direct from your battery or via an inverter (as this also runs from your battery). This is a worked example only below. Keep in mind that this example would be very heavy power use, more typical of a large caravan or motorhome but the process and calculations are the same. You also need to get the exact power use from your appliances too. These are just my examples…they all do vary a lot.
Once you know what you want to run and whether you will run it during the day or night or both, you need to calculate two things.
- Total daily amp hours (Ah) of use. Ah = Amps x No. hours use. This is used to calculate the size of your solar panels – In the above example 170Ah
- Total night time only use. This helps you to size your battery – 85Ah
Things to consider...
Once you have these values, you can then go about sizing your Solar Panels. Weather conditions (clouds and temperature), location (trees, steep/low terrain) and placement of panels all affect the performance of your solar panels.
As temperature goes up, the voltage output of your panels will drop. Therefore, most solar panels for 12v systems actually start at around 17-21V. In temperatures, say 40oC, you will see voltages drop as much as 2-3volts. If you started at 15V and dropped to 12V, then you wouldn’t have enough volts to charge your battery. Let’s take an AGM battery for example. Fully charged at rest it will be about 12.6-12.7v, or what they call Nominal “12v”, but during charging it needs 14.4v. If your panel was supplying less than 14.4v then it would never achieve a full charge.
Clouds, shade and hrs of direct sun say from hills either side of you will all affect the number of hrs of sun, or more specifically UV light you get on your panels. The other key thing is the angle your panel is facing to the sun. You aren’t going to move your panel to face the optimum angle every 20mins of the day. You are on holidays and want to set and forget (at least for a few hours). This means that your panels are rarely producing full power.
Due to all of the above reasons, it is recommended to expect a maximum of around 5-6 hours of sun light per day, meaning your solar system needs to produce your full allocation of power for the whole day in less than 6 hrs. Using the above example of 170Ah, it needs to produce 28.45 Amps per hour for 6 hours. To work out the size of the panel we need, we simply take the 28.45 x 18V (average voltage of a solar panel) = 512W. Now as I said before, this is a high power example, more typical of a large offgrid caravan or motorhome, but the approach is the same. Just substitute your loads and perform the same calculation.
How to choose a panel?
Everyone has a budget. That’s a fact, which is based on personal financial circumstances and how often you want to use the products you, but the old adage “You get what you pay for” and “Poor man buys twice” is very true when it comes to 12v equipment. We’ve been there, done that and the extensive list of cheap crap out there promising the world, but never delivering was what led us to start Offroad Living in the first place. High price doesn’t always mean a better quality product, but a very low price will always mean a much lower standard. Cheaper materials, less quality control (or none), less inclusions in the specification and generally lower performance and shorter life expectancy.
If your budget is tight, buy the best quality panel you can for your budget. If you have a very low budget, but some time up your sleeve, save for a bit longer. Alternatively, check out stores with options to pay in installments (eg. AfterPay, ZipPay). Focus on panel performance rather than digital displays or wireless connectivity. Get the highest max amp output you can get and make sure it comes with a good quality support frame.
What about eBay?
Buying on price alone or based on eBay feedback is a slippery slope. On eBay most people give feedback when the product arrives. It arrives quickly and at first glance it looks good and you give positive 100% feedback. Then it breaks the next week, but you cannot do anything to tell others it didn’t last the first trip.
One tactic we’ve used to determine whether we want to purchase an item is to contact the seller - ask a question you know the answer to, ask one you don’t. Guage responses, their response time, their English levels. ask them to confirm their return policies and warranty process. Make sure you understand how the seller is going to behave if things go wrong. Even with the highest quality products, things can go wrong. Having a seller that communicates well and is contactable is key to having confidence in the knowledge that you won’t be back purchasing again in 6 months time. Warranties don’t mean much if the seller isn’t a business and disappears the next day.
If your budget extends a little further, here are some things to think about.
- Physical dimensions and weight of panel (how much room will it take up in my car and how heavy is it).
- Does it have a multi stage charge controller or simple voltage regulator?
- What about the bag? Is it just a thin dust cover or a rigid protective carry case to protect it in transit?
- Heavy duty cables to reduce the potential voltage drop? The thicker the cable, the better the performance.
- Look for the quality of the cable terminations. Properly insulated with heat shrink over connections and integrated fuses.
- Is the charge controller dust and waterproof (IP67).
The answer to most of these questions on the cheap eBay specials is a big fat and sometimes very dodgy “NO”. I even came across a customer 2 weeks ago that bought a cheap 160W eBay special that never performed how it should. I tested it for her. It produced less than 4amps in full sun. I checked the details only to find that she had actually been supplied with what would barely be a 120W panel (the manual confirmed it from the dimensions and specifications). The seller refused to do anything about it. Like I said “DODGY!”
A final note in my recommendation. Stick to Mono-crystalline panels. They are smaller in size (footprint), have higher efficiency levels. They also operate better in lower UV levels and are only marginally more expensive than Poly crystalline (old technology).
If this is just too much to take in or you cannot work out the calculations yourself and need help, well that’s where we come in. Please don’t hesitate to contact us in the comments section below, on our Facebook page @Offroadliving, by email (email@example.com) or start a new post on our Facebook group “Offroad Living Tribe - Australian camping, 4WDing and caravanning”.